Add a fitted.bfast() method

Right now, running `fitted()` on a `bfast` object returns `NULL` (that's returned for any unknown class). It would be better if it returned `x$output[[iteration]]$St + x$output[[iteration]]$Tt` instead.

Start by locating the existing fitted() generic and the bfast object structure, then inspect how iteration, output, St, and Tt are represented. Add the bfast-specific fitted() behavior so it returns x$output[[iteration]]$St + x$output[[iteration]]$Tt, and verify that calling fitted() on a bfast object no longer returns NULL.

Written by the indexing model from the issue text.